Energy-resolving photon counting detectors are based on direct conversion materials, such as Cadmium Zinc Telluride (CdZnTe, also known as CZT) or Cadmium Telluride (CdTe). Direct conversion materials are compound semiconductors that often exhibit non-negligible unwanted which cause false information and/or increased noise or loss of resolution. For instance, a detector pixel may receive some charge intended for a neighboring pixel (charge sharing) or it may exhibit current running through the semiconductor material even when no radiation is emitted towards the semiconductor (dark current). The dark current ranges from a few nA per pixel to several tens of nA per pixel depending on the type of electrodes (e.g. blocking or Ohmic electrodes) and sensor resistivity. Said dark current is highly dependent on temperature, typically in an exponential function, mainly caused by increased thermal equilibrium densities of conduction band electrons and valence band holes with increasing temperature.
Particularly CZT exhibits a number of undesirable artefacts with various causes which have been continuously improved upon in recent years. To address some of these artefacts (e.g. photoconductive gain) Baseline Restoration (BLR) circuits for example are required. Such circuits also compensate for the dark current and slow fluctuations caused by temperature changes. BLR circuits however also cause a number of artefacts themselves in the context of high rate applications. As soon as CZT does not exhibit any flux dependent excess current (other than photo-current) the use of a BLR is not any longer justifiable given the additional artifacts that it causes and particularly considering that dealing with such circuit imperfections (sensitivity to induction, pile-up, etc.) requires development of significantly more complex circuits.
An acceptable solution for the aforementioned dark-current dependency on temperature would be highly desirable. A change in dark current causes a baseline shift which will in turn cause an error in energy estimation. Although the detector temperature is generally regulated, a temperature margin below +/−1° C. may not be ensured. This may cause energy drifts exceeding 2 keV depending on implementation.
Current solutions include for instance grid-switch sampling: synchronizing the sampling of the baseline shift to a short period where the x-rays are off, distributed throughout a complete scan, as is for instance known from US 2013/0284940 A1. This solution requires special, advanced X-ray tube and generator functionality.
Another solution may be pre-scan sampling: before starting a scan, the baseline is sampled and the dark-current is compensated for. For long scans however the temperature may deviate, which causes an energy estimation error.
A third potential solution is AC coupling: this completely eliminates sufficiently low frequency changes. It however requires a large decoupling capacitor and input biasing resistor, not compatible with the high level of monolithic integration needed. It also requires a BLR or a baseline holder (BLH) to re-establish a reference.
US2011/0248175A1 discloses a temperature compensation circuit for nuclear detectors that include reference APDs placed on the detector surface together with sensor pixels.